1. Field of the Invention
The present invention is directed to a process and a corresponding device for filtering plastics in an injection molding machine in which a holding device receiving a filter element is arranged between a plasticizing unit and a mold cavity.
2. Description of the Related Art
Kunststoffe [Plastics] 87 (1997), pages 154-159, discloses a screen system (page 156) in which the screening locations are arranged in a rectangular slide (cassette) or a round bolt. Changing is initiated manually or automatically via a hydraulic drive when a defined pressure limit is reached by displacing the screen surface carrier.
Screen changing systems of the above-mentioned type are used in plastics processing and, in this case, especially in extrusion. Problems occur in this respect precisely in changing systems which are not expressly constant with respect to pressure, since the continuous insertion and exchange of the screen surface involves sometimes enormous pressure fluctuations in front of the filter.
German reference DE 44 19 284 C1 discloses a filter device for plastic melts in which screen inserts are arranged in a power-operated displaceable slide. Seals are arranged at both sides of the slide, wherein at least one of the seals has a sealing body made of metal which encloses a hollow space that is filled with a substance whose thermal expansion coefficient exceeds that of the material of the sealing body. When the seal is inserted, its sealing body which is made of metal, for example, high-grade steel, is relaxed in practice. As a result of heating by the plastic melt flowing through, the sealing body, including the material with which it is filled, expands thermally so that the end face of the seal lies firmly against the slide in a sealing manner during operation after the thermal expansion or deformation.
A disadvantage in this type of seal is its dependence on the pressure and on the temperature of the processed plastic substance such that at low pressure and low temperature leaks occur and at high pressure and high temperature undesirably high area pressing occurs in the support surfaces.